GRAFTON, Mass. — A popular dog treat may be adding too many calories to the diets of dogs while exposing owners to harmful bacteria, researchers at the Tufts University Cummings School of Veterinary Medicine have found.

The treat is known as a bully or pizzle stick, made from the uncooked, dried penis of a bull or steer. Many owners don't realize what part of the bull they were feeding their pets, said the study, co-authored by researchers at the Cummings School and the University of Guelph, a Canadian university.

"While calorie information isn’t currently required on pet treats or most pet foods, these findings reinforce that veterinarians and pet owners need to be aware of pet treats like these bully sticks as a source of calories in a dog’s diet," said Dr. Lisa M. Freeman, a Cummings School professor of nutrition who is board certified by the American College of Veterinary Nutrition.

The study, published in the January issue of the Canadian Veterinary Journal, examined 26 bully sticks purchased from retailers in the United States and Canada and made by different manufacturers.

A random subset of the bully sticks was tested for caloric content. They contained 9 to 22 calories per inch, meaning the average 6-inch stick packed 88 calories — 9 percent of the daily calorie requirements for a 50-pound dog and 30 percent of the daily calorie requirements for a 10-pound dog.

"With obesity in pets on the rise, it is important for pet owners to factor in not only their dog's food, but also treats and table food," Freeman said.

All 26 treats were tested for bacterial contaminants. One (4 percent) of the sticks was contaminated with Clostridium difficile; one (4 percent) was contaminated with methicillin-resistant Staphylococcus aureus (MRSA), a type of staph bacteria that is resistant to certain antibiotics; and seven (27 percent) were contaminated with E.coli, including one tetracycline-resistant sample.

The study stressed the sample size was small and not all of the bacterial strains have been known to infect humans. The researchers advised pet owners to wash their hands after touching the treats, as they should with all raw meat products. Very young, elderly, pregnant, immunocompromised and other high-risk individuals should avoid all contact with raw animal product-based treats.

In a web-based survey of 852 adults, mostly female dog owners, from 44 states and six countries, most respondents did not know the contents of bully sticks and 23 percent of the respondents fed their dogs bully sticks.

"We were surprised at the clear misconceptions pet owners and veterinarians have with pet foods and many of the popular raw animal-product based pet treats currently on the market," Freeman said. "For example, 71 percent of people feeding bully sticks to their pets stated they avoid byproducts in pet foods, yet bully sticks are, for all intents and purposes, an animal byproduct."

Further research with a larger sample size is needed to determine whether the calorie content and contamination rate found in this study is representative of all bully sticks or other types of pet treats, according to the authors.